In the operation of nuclear reactors, it is customary to remove fuel assemblies after their energy has been depleted down to a predetermined level. In the commercial nuclear industry, fuel assemblies are typically an assemblage of long, hollow, zircaloy tubes filled with enriched uranium. Upon depletion and subsequent removal, spent nuclear fuel is still highly radioactive and produces considerable heat, requiring that great care be taken in its packaging, transporting, and storing. Specifically, spent nuclear fuel emits extremely dangerous neutrons and gamma photons. It is imperative that these neutrons and gamma photons be contained at all times.
Upon defueling a nuclear reactor, spent nuclear fuel is placed in a canister that is submerged in a storage pool. The storage pool facilitates cooling of the spent nuclear fuel and provides radiation shielding that helps contain the emitted neutrons and gamma photons. Generally, canisters are cylindrical steel containers with flat bottoms. A typical canister can hold approximately 24 PWR fuel assemblies or 60 BWR fuel assemblies. When fully loaded with spent nuclear fuel, a canister weighs approximately 45 tons. However, a canister alone does not provide adequate containment of the neutrons and gamma photons emitted by the spent nuclear fuel contained therein. As such, a loaded canister cannot be further transported from the storage pool without some additional radiation shielding. Because it is preferable to store spent nuclear fuel in is “dry state,” the canister must eventually be removed from the storage pool. As such, the canisters need additional radiation shielding during transport and long-term dry storage of the spent nuclear fuel.
In state of the art facilities, additional radiation shielding is achieved by placing the loaded canisters in large cylindrical containers called casks. There are two types of casks used in the industry, storage casks and transfer casks. A transfer cask is used to transport, canisters of spent nuclear fuel from location to location while a storage cask is used to store spent nuclear fuel in the “dry state” for long periods of time. Both transfer casks and storage casks are designed to shield the environment from the neutron and gamma radiation emitted by the spent nuclear fuel through the use of two principles.
First, the gamma radiation emitted by spent nuclear fuel is blocked by placing mass in its way, the greater the density and thickness of the blocking mass, the more effective the attenuation of the gamma radiation. Examples of effective gamma absorbing materials are concrete, lead, and steel. Second, the neutrons emitted by spent nuclear fuel are blocked by placing a neutron absorbing material in their path. Any material rich in hydrogen serves as an effective neutron shield. One such example of an effective neutron absorbing material is water. Other types of material may also serve as an effective neutron shield, such as aluminum trihydrate mixed with boron carbide in an epoxy resin.
Guided by the above principles, storage casks are designed to be large, heavy structures made of steel, lead, concrete and an environmentally suitable hydrogenous material. However, because the focus in designing a storage cask is to provide adequate radiation shielding for the long-term storage of spent nuclear fuel, size and weight are often secondary considerations (if considered at all). As a result of maximizing the thickness of the gamma and neutron absorbing materials, the weight and size of storage casks often cause problems associated with lifting and handling. Typically, storage casks weigh approximately 150 tons and have a height greater than 15 ft. A common problem is that storage casks are often too heavy for the capacity of most nuclear power plant cranes and as such cannot be lifted. Another common problem is that storage casks are too large to be placed in storage pools. Thus, in order to store spent nuclear fuel in a storage cask, a loaded canister must be removed from the storage pool, prepared in a staging area, and transported to the storage cask. Additional radiation shielding is needed throughout all stages of this procedure.
Removal from the storage pool and transport of the loaded canister to the storage cask is facilitated by a transfer cask. In facilities utilizing transfer casks to transport loaded canisters, an empty canister is placed into an open transfer cask. The canister and transfer cask are then submerged in the storage pool. As each assembly of spent nuclear fuel is depleted, it is removed from the reactor and lowered into the storage pool and placed in the submerged canister (which is within the transfer cask). The loaded canister is then fitted with its lid, enclosing the spent nuclear fuel and water from the pool within. The enclosed water provides neutron radiation shielding for the spent nuclear fuel once the transfer cask is removed from the pool. The canister and transfer cask are then removed from the pool by a crane and set down in a staging area to prepare the spent nuclear fuel for storage in the “dry state.” Once in the staging area, the water contained in the canister is pumped out of the canister. This is called dewatering. Once dewatered, the spent nuclear fuel is allowed to dry. Once dry, the canister is back-filled with an inert gas such as helium. The canister is then sealed and the canister and the transfer cask are once again lifted by the plant's crane and transported to the storage cask. The transfer cask is placed atop the storage cask and the canister is lowered through a bottom opening in the transfer cask into the storage cask.
Because a transfer cask must be lifted and handled by a plant's crane (or other equipment), transfer casks are designed to be a smaller and lighter than storage casks. A transfer cask must be small enough to fit in a storage pool and light enough so that, when it is loaded with a canister of spent nuclear fuel, its weight does not exceed the crane's rated weight limit. Additionally, a transfer cask must still perform the important function of providing adequate radiation shielding for both the neutron and gamma radiation emitted by the enclosed spent nuclear fuel. As such, transfer casks are made of a gamma absorbing material such as lead and contain a neutron absorbing material. While the pool water sealed in the canister does provide some neutron shielding, this water is eventually drained at the staging area. As such, many transfer casks have either a separate layer of neutron absorbing material or have an annulus filled with water that surrounds the cavity of the transfer cask in which the loaded canister is located.
The allowable weight of a transfer cask is limited by the lifting capacity of the plant's crane (or other lifting equipment). The load handled by the crane includes not only the weight of the transfer cask itself, but also the weight of the transfer casks payload (i.e., the canister and its contents). A transfer cask must be designed so that the total load handled by the crane during all handling evolutions does not exceed the crane's rated weight limit, which is typically in the range of 100 125 tons. As such, the permissible weight of a transfer cask is equal to the rated capacity of the plant crane less the weight of its payload. Moreover, when the combined weight of a transfer cask and its payload is equal to the rated lifting capacity of the plant crane, the possible radiation shielding that can be provided by a transfer cask is at a maximum for that particular payload.
Because the weight of the transfer cask's payload varies during the different stages of the transport procedure, the permissible weight of the transfer cask is equal to the rated capacity of the plant crane less the weight of the transfer cask's maximum payload at any lifting step. The weight of the transfer cask's payload is at a maximum when the transfer cask and canister are lifted out of the storage pool, at which time the canister is full of spent nuclear fuel and water. Thus, according to prior art methods, it is at this stage that the permissible weight of a transfer cask is calculated. The transfer cask is then constructed using this permissible weight as a design limitation.
The design limitations of the weight of the transfer cask have other implications. For example, in the present state-of-the-art, a small annulus is formed between the canister and the transfer cask cavity which is protected from contamination while in the pool by a seal and by employing a high integrity gasketed joint between the bottom lid and the cask body. When outside the pool, the annulus is typically filled and continually flushed with demineralized water, which provides an effective means to remove the decay heat generated by the canister and thus keep its fuel cladding temperature within regulatory limits. The circulating water annulus cooling system utilizes flowing water in a closed loop as the medium to extract heat from the wall of the canister. The heated water, in turn, is cooled by an air cooler or some other heat exchanger. However, the heat transmission capability provided by the water in the annulus between the canister and the cask is inconvenient and cumbersome to maintain when the cask has to be transported on the haul path on its way to the storage facility.
Other features of the transfer cask that do not directly pertain to radiation shielding may be compromised or entirely neglected so that the permissible weight of the transfer cask, when loaded with the payload, is no more than the rated capacity of the plant crane.